1. Field of the Invention
The present invention relates generally to a projection exposure and method for use with a photolithography process for manufacturing a semiconductor device, etc. and a projection optical system thereof and, more particularly, to a projection exposure apparatus and method and a projection optical system that are suitable for an application to a case where a periodical pattern such as, e.g., a line-and-space pattern is transferred on a photosensitive substrate.
2. Related Background Art
A projection optical system for a projection exposure apparatus used when forming hyperfine patterns of, e.g., a semiconductor integrated circuit, a liquid crystal display, etc. in a photolithography process is required to exhibit an extremely high image forming characteristic in order to transfer a variety of different patterns on a reticle (or photomask) onto a photosensitive substrate, such as a wafers, with a fidelity.
Generally, important factors for transferring the hyperfine reticle patterns onto the photosensitive substrate through the projection optical system with fidelity are depth of focus and resolution of the projection optical system. The resolution may be improved simply by increasing a numerical aperture of the projection optical system.
Herein, a relationship between the resolution and the numerical aperture will be explained. In this connection, the patterns of the normal integrated circuit are arrayed mainly in two directions orthogonal to each other, particularly, in the lengthwise (short-side direction) or the cross-wise direction (long-side direction) that are parallel respective sides of shot areas on the wafer. However, there may be a layer of the integrated circuit including very few patterns (oblique patterns) arrayed in an oblique direction or in which there is no oblique pattern at all. Herein, an explanation will be given by taking an example wherein periodic patterns arrayed in the lengthwise direction on, e.g., the reticle are irradiated with illumination light beams for exposure.
Some of the illumination light beams falling on the reticle are partly blocked by the periodic patterns thereof, while the illumination light beams penetrating the reticles contain not only the light components (0th-order) directly passing therethrough but also the light components (diffracted light) diffracted and deflected by the periodic patterns. These two kinds of light components pass through the projection optical system and interfere with each other on the photosensitive substrate to form bright-and-dark fringes, thereby transferring the periodic patterns. Herein, a diffracting angle xcex8 of the diffracted light generated from the reticle is a function of a wavelength xcex of the illumination light and a pitch P of the periodic patterns, and, with respect to (n)th-order (n is an integer) diffracted light, the following formula (1) is established:
sin xcex8=nxcex/Pxe2x80x83xe2x80x83(1)
That is, the diffracting angle xcex8 becomes larger as the patterns are hyperfiner. Hence, there has been developed the projection optical system (projection lens system and projection mirror system) having a larger numerical aperture to transfer the much hyperfiner patterns with a high accuracy by increasing the resolution. Then, a stop (aperture stop) for defining the numerical aperture is disposed on a pupil plane (Fourier transform plane with respect to the reticle pattern surface) of the projection optical system. The aperture of the conventional aperture stop is of a circular shape, and especially none of those aperture stops has a directivity.
When the circular aperture stop is provided on the pupil plane of the projection optical system however, a maximum diffracting angle at which the light passes through the projection optical system differs in accordance with an incident angle in a direction perpendicular to an arraying direction of the periodic patterns.
FIGS. 12 and 13 show a relationship between the incident angle of the illumination light upon a reticle 300 and a circular aperture stop 302 of the projection optical system.
As illustrated in FIG. 12, when the illumination light (vertical illumination light) is incident, parallel to an optical axis, on periodic patterns 304 arrayed in the X-direction on the reticle 300, the diffracted light of an exit angle xcex80 corresponding to a numerical aperture (termed NA) of the projection optical system is capable of passing through the projection optical system. That is, in the case of the vertical illumination light, a sine of the effective exit angle xcex80 of the diffracted light is substantially equal to NA.
However, when the illumination light is incident on the reticle 300 with an inclination in the Y-direction to the optical axis AX, as illustrated in FIG. 13, a quantity of the diffracted light capable of passing through the projection optical system is smaller than the vertical illumination light. Namely, when the illumination light is incident with an inclination in the Y-direction, the sine of an effective exit angle xcex81 of the diffracted light is smaller than NA.
For instance, if the sine of the incident angle of the illumination light in the Y-direction perpendicular to the arraying direction of the patterns is 0.7 times the numerical aperture on the incident side (NA of the illumination optical system), it follows that the sine of the effective exit angle of the diffracted light becomes 0.7 times (=0.7 NA) the sine of the effective exit angle of the diffracted light in the case of the vertical illumination light.
Further, in the case of a the larger numerical aperture, a range of the incident angle on the wafer is also increased, and hence a phase difference between the light beams for forming the image due to a difference between the incident angles is increased. This decreases an effect in which the light beams interfere and thus enhance each other, even in, e.g., a bright-image position, and there is a larger degree of decrease in resultant image contrasts. It has been gradually recognized in recent years that an excessive numerical aperture is not desirable, in order to obtain a sufficient depth of focus.
As explained above, in the projection optical system including the circular aperture stop, the effective exit angle differs depending on the incident angle of the illumination light incident on the projection optical system. If the sine of the incident angle optimizes the numerical aperture of the projection optical system with respect to the illumination light, that is, approximately, e.g., 0.7 times the numerical aperture, on the incidence side, the numerical aperture is excessive for the vertical illumination light. For this reason, even when in the case of an expensive projection optical system having a large numerical aperture, and when having such a circular aperture stop that the exit angle of the light capable of passing through the projection optical system differs depending on the incident angle, there is a disadvantage in that the performance thereof is insufficient.
Further, though the great majority of patterns of the actual integrated circuit are formed of the patterns in the lengthwise and crosswise-directions, hyperfine patterns may also exist in oblique directions. In such a case, it is also desirable that the numerical aperture be not varied depending on the incident angle in an oblique direction. Hence, there is needed a projection optical system capable of high performance even with patterns in oblique directions as the necessity arises.
It is a primary object of the present invention, which was devised in view of the above points, to provide a projection optical system having a proper numerical aperture on the whole with less difference between effective exit angles dependent on incident angles in a direction perpendicular to a predetermined direction of illumination light and a projection exposure apparatus and method employing this projection optical system.
A projection optical system according to the present invention projects and forms an image of a pattern (20) arrayed on a first surface (14) and exhibiting a periodicity in a predetermined direction (X- or Y-direction) onto a second surface (17). A stop (16; 16A) includes an aperture (106) showing a line symmetry with respect to a symmetric axis (Y1, X1) intersecting an optical axis (AX) of the projection optical system and extending in a direction (Y- or X-direction) orthogonal to the predetermined direction and having an outline (109) at least a part of which are rectilinear portions. This stop is disposed on a Fourier transform plane of the first surface (14) within the projection optical system or a plane in the vicinity thereof.
In this case, the outline (109) of the stop (16; 16A) preferably has rectilinear portions parallel to the symmetric axis (Y1, X1).
Further, the stop (16; 16A) preferably includes a plurality of movable blades (101A, 101B) having rectilinear edges each for defining the outline (109) of the aperture (106).
Moreover, a projection exposure apparatus according to the present invention is equipped with a projection optical system. A mask (14) formed with a pattern (20) to be transferred is disposed on a first surface thereof, while a photosensitive substrate (17) is disposed on a second surface thereof. The projection exposure apparatus comprises an illumination optical system (1-6, 8a, 8c, 8d, 9-13) for Kxc3x6hler-illuminating the mask with beams of exposure illumination light that are emitted from such a surface light source that its outline corresponding to the rectilinear portions of the outline (109) of the aperture (106) of the stop (16) takes a rectilinear shape.
In this case, the surface light source of the illumination optical system takes a rectangular frame-like configuration (8d) as one example thereof.
According to the projection optical system of the present invention described above, when projecting the pattern (20V) arrayed at a predetermined pitch in, e.g., the X-direction, the stop (FIG. 5B) showing line symmetry with respect to the symmetric axis (Y1) parallel to the Y-direction (perpendicular to the X-direction) and having the rectilinear portions is disposed in the vicinity of the Fourier transform plane surface of the projection optical system (15). With this arrangement, the projection optical system transmits substantially the same quantity of diffracted light from the pattern (20V) due to the illumination light inclined in the Y-direction as the diffracted light due to the vertical incidence light. That is, there is only a small difference between the effective exit angles. Note that if the aperture of the aperture stop does not have a circular shape, the direction of the diffracted light generated from the pattern differs depending on a direction of the pattern to be transferred. Therefore, it follows that the numerical aperture changes based on the pattern direction. Herein, if the numerical aperture differs, the range of the incident angle on the image surface of the diffracted light for forming the image differs if the numerical aperture is different, and, therefore, image forming characteristic such as image contrast and depth of focus become different. For this reason, when adjusting the image forming characteristic of the projection optical system, an undesirable aperture stop is one having an aperture of a square shape or the such that image forming performance differs depending on the pattern direction. A desirable aperture stop is therefore one having a circular aperture when adjusting the projection optical system and one having a square aperture or the like when an actual exposure is conducted.
Further, if the outline (109; 103a) of the aperture of the stop (16; 16A) includes rectilinear portions parallel to the symmetric axis thereof, even when the incident angle differ, the diffracted light generated from the pattern arrayed in the predetermined direction on the mask substantially uniformly travels through the projection optical system. Hence, there is almost no difference between the effective exit angles, and the numerical aperture is optimized.
Also, when the stop (16; 16A) includes the plurality of movable blades (101A, 101B) having the rectilinear edges each for defining the outline (109) of the aperture (106), the shape of the aperture can be varied corresponding to, e.g., the actual exposure and the adjustment of the projection optical system.
Further, in the projection exposure apparatus incorporating the projection optical system (15) according to the present invention, the range of the effective diffracting angle does not change even if the incident angle of the illumination light incident on the projection optical system (15) changes, and the numerical aperture of the projection optical system (15) is optimized, thereby obtaining a good image forming characteristic. Moreover, in the case of the Kxc3x6hler illumination, the surface light source of the illumination optical system is formed on the pupil plane of the projection optical system, and hence at least a part of the outline of the surface light source also is formed in the rectilinear shape in accordance with the configuration of the stop in the vicinity of the pupil plane thereof, whereby image forming characteristics are enhanced, and there is a reduced amount of harmful image forming light beams.
Also, if the surface light source of the illumination optical system assumes a rectangular frame-like shape (8d), there is conducted illumination in which a normal annular illumination method is optimized for the patterns in the lengthwise-direction and crosswise-direction.